The invention relates to gamma-ray detection, and in particular to gamma-ray detection for determining the direction to a source of gamma-rays.
There are a number of situations in which it can be important to be able to quickly determine the level of radiation in the environment, the nature of the isotope producing the radiation, and the direction from which the radiation is coming. For example, this information can be of great benefit to emergency staff entering a ‘disorganised’ nuclear environment, for screening personnel and cargo in order to police the illicit trafficking of radioactive materials, and for general searching for ‘orphaned’ sources of radiation.
Gamma-ray detectors with the ability to measure the intensity of radiation in the environment and to identify the nature of the source emitting the radiation (from spectroscopic information) are widely available, for example the GR-135 Exploranium® from the SAIC Corporation. However, to determine the direction to the source using these detectors the user must rely on dose-rate trends. This is done by moving the detector and noting how the measured intensity changes. An increase in measured intensity indicates the motion is towards the source. A decreases in measured radiation indicates the motion is away from the source. The location of the source may thus be found by trial and error. A problem with this approach is than it can be slow and unreliable, and so results in an increased radiation exposure to the user seeking to identify the source.
FIG. 1 schematically shows in section view a known hand-held gamma-ray detector 2 capable of determining the intensity, nature and direction of a radioactive source [1]. The detector 2 comprises a spectrometer component 4 and a separate direction finding component 6. The detector 2 is powered by batteries 8 and includes a dock 10 for receiving a personal data assistant (PDA) type device 12 configured to control the detector and display results to a user.
The spectrometer component 4 is responsible for determining the intensity of radiation in the environment and its spectrum. From the spectrum, the nature of the source can be determined. The spectrometer component 4 comprises a conventional small-volume Cerium-doped Lanthanum Bromide (LaBr3(Ce)) crystal scintillator coupled to a photo-multiplier tube.
The direction finding component 6 is responsible for determining the direction from which the radiation in coming, and hence the direction to the source. The direction finding component 6 comprises a cluster of four Geiger-Muller tubes separated from one another by lead shielding. The count rates in the Geiger-Muller tubes which are shielded from the source by the lead shielding will be lower. Accordingly, the direction to the source can be determined from the relative count rates seen in the Geiger-Muller tubes. Experiments have shown that the detector 2 shown in FIG. 1 is capable of localising a 130 millicurie Cesium-137 source at a distance of 2 m (resulting in a dose rate of around 180 μSv/hr at the detector) to within +/−7.5 degrees.
Although the detector 2 shown in FIG. 1 is able to provide useful information regarding the intensity, nature and direction of a gamma-ray source, it has some drawbacks. For example, the direction finding component 6 requires lead shielding and this makes the detector relatively heavy. This can be particularly important for a hand-held detector because the increased weight means it becomes less wieldy, especially if it is to be held for long periods. Furthermore, the direction finding component 6 takes up space in the detector housing. This means for a given characteristic size of detector, e.g., a size that can be comfortably hand held, there is less space available for the spectrometer component than there would be in a dedicated hand-held spectrometer. This can be a problem because the sensitivity of a scintillator-based spectrometer is closely tied to the volume of the scintillator material used. Accordingly, the limited space available for the spectrometer component 4 in the detector shown in FIG. 1 means the detector is overall less sensitive to radiation compared to a dedicated spectrometer having a similar characteristic size. Furthermore still, the inclusion of the separate direction finding component 6 adds to the overall complexity of the detector, for example because a separate high voltage supply and processing circuitry for the Geiger-Muller tubes are required.
There is therefore a need for a gamma-ray detector that enables a user to quickly determine the level of radiation in the environment, the nature of the isotope producing the radiation, and the direction from which the radiation is coming, but which is less complex and can be made lighter and less bulky than the detector 2 shown in FIG. 1.